1. Field of the Invention
This invention relates to a method for forming a crystal which is used for forming, for example, electronic elements such as semiconductor integrated circuit, optical integrated circuit, magnetic circuit, etc., optical elements, magnetic elements, piezoelectric elements, or surface acoustic elements, etc.
2. Related Background Art
In the prior art, thin films comprising single crystals to be used for preparation of electronic elements or optical elements, etc. constituted of semiconductors and insulating materials have been formed by epitaxial growth on single crystalline substrates. For example, on a Si single crystal (silicon wafer), Si, Ge, GaAs, etc. have been known to be epitaxially grown from liquid phase, gas phase or solid phase, and also on a GaAs single crystal substrate, a single crystal such as GaAs, GaAlAs, etc. has been known to be epitaxially grown. By use of the single crystalline semiconductor thin film thus formed, semiconductor elements and integrated circuits, electroluminescent elements such as semiconductor laser, LED, etc. are prepared.
Also, recently, researches and developments of ultra-high speed transistors by use of two-dimensional electronic gas, ultra-lattice elements utilizing quantum well, etc. have been extensively performed, and these were rendered possible by high precision epitaxial technique such as MBE (molecular beam epitaxy) by use of ultra-high vacuum, MOCVD (metalloorganic chemical vapor deposition), etc.
However, in such epitaxial growth on a single crystal substrate, it is necessary to match the lattice constant and the thermal expansion coefficient between the single crystal material of the substrate and the epitaxial growth layer. If this matching is insufficient, crystal defects such as lattice defect, etc. will develop in the epitaxial layer. Also, the elements constituting the substrate may be diffused into the epitaxial layer.
Thus, it can be understood that the method of forming a thin film single crystal according to epitaxial growth depends greatly on its substrate material. Mathews et al have examined combinations of the substrate material and the epitaxial growth layer (EPITAXIAL GROWTH, Academic Press, New York, 1975, ed. by J. W. Mathews).
The size of a single crystal substrate is presently about 6 inches for Si wafer, and enlargement of GaAs and sapphire substrate has been further delayed. In addition, since a single crystal substrate is high in production cost, the cost per chip inevitably becomes higher.
Thus, for permitting a single crystal of good quality which can be prepared according to the method of the prior art, there has been the problem that the kind of the substrate material is limited to a very narrow scope, and the degree of freedom during design and preparation is small.
On the other hand, researches and developments for three-dimensional integrated circuits which accomplish high integration and multi-function by laminating semiconductor elements in the normal direction of the substrate have been extensively done in recent years. Researches and developments for enlarged area semiconductor devices such as solar battery, switching transistor of liquid crystal display device, etc. in which elements are arranged in an array on an inexpensive glass are also becoming increasingly popular year by year.
What is common to both of these is formation of a thin film single crystal on an amorphous insulating material, and a technique for forming an electronic element such as a transistor, thereon is required there. Particularly a technique for forming a single crystal semiconductor of high quality on an amorphous insulating material has been desired.
Generally speaking, when a thin film is deposited on an amorphous insulating material substrate such as SiO.sub.2, due to deficiency of long distance order of the substrate material, the crystalline structure of the deposited film will not become single crystal, but become amorphous or polycrystalline. Accordingly, the amorphous film is a film under the state, wherein although the short distance order of the nearest atom is preserved, there is no long distance order, and the polycrystalline film is a film in which single crystals having no specific crystal direction are aggregated as separate crystals with grain boundaries.
For example, when Si is formed by the hot CVD method on amorphous SiO.sub.2, if the deposition temperature is about 580.degree. C. or lower, it becomes amorphous silicon, whereas it becomes polycrystalline silicon with grain sizes ranging between hundreds to thousands A at a high temperature. However, the grain sizes of polycrystalline silicon and the distribution thereof will vary greatly depending on the formation method.
Further, in the method of forming a polycrystalline thin film of large grain size, by melting and solidifying an amorphous film or a polycrystalline film with thermal energy by use of a laser, a rod-shaped heater, etc., a polycrystalline film with a large grain size of about micron or millimeter is obtained (Single Crystal silicon on non-single-crystal insulators, Journal of Crystal Groth vol. 63, No. 3, October, 1983, edited by G. W. Cullen).
Referring now to an example in which an electron element such as transistor, is formed on the thin film of each crystal structure thus formed, electron mobility is measured from it characteristics and compared with the electron mobility when using a polycrystalline silicon. The electron mobility in the polycrystalline silicon having a grain size of several .mu.m to several mm formed by melting and solidification is about the same as that in the case of single crystalline silicon, the electron mobility in the polycrystalline silicon having a grain size distribution of some hundreds to some thousands A is about 10.sup.-3 of that of single crystal silicon, and an electron mobility of about 2.times.10.sup.-4 of that of single crystal silicon is obtained in amorphous silicon.
From these results, it can be understood that there is a great difference in electrical characteristics between the element formed within the single crystal region and the element formed as bridging across the grain boundary. Thus, the deposited film on amorphous surface obtained in the prior art method becomes a non-single-crystalline structure such as amorphous structure or a polycrystal, having grain size distribution, and the element prepared thereon is greatly inferior in its performance as compared with the element prepared in a single crystal. Accordingly, the uses are now still limited to simple switching elements, solar batteries, photoelectric converting elements, etc.
Also, the method for forming a polycrystalline thin film with a large rain size by melting and solidification as described above requires scanning of an energy beam on an amorphous or polycrystalline thin film for each wafer, and therefore requires an enormous time for making the grain size larger resulting in poor bulk productivity. Thus there arise the problems of diffusion of the impurity by the heat applied for melting and insuitability for enlargement of area.
As described above, according to the crystal formation method of the prior art, three-dimensional integration or enlargement of area could not be done with ease, practical application to a device has been difficult, and a single crystal required for preparation of a device having excellent characteristics could not be formed easily and at low cost.
For solving this problem, the present Applicant has proposed a crystal growth method (European Laid-open Patent Publication 0:224,081, published on Nov. 4, 1987) and a crystal growth method utilizing agglomeration by heat treatment (European Laid-open Patent Publication 0,306,153 published on Mar. 8, 1989), etc.